The invention relates to storage systems including a plurality of storage units, each of which rolls on tracks and is powered for movement by an electric motor. A storage system of this type is disclosed in U.S. Patent Application Ser. No. 69,012 filed on Sept. 2, 1970. In the prior system, a central control circuit governed the movements of the separate units. In the present invention all the control circuitry for each unit is carried by the units themselves, thereby making each unit an independent modular element. Each unit may be removed or inserted into the stack assembly without changing or altering the other units. The circuitry in each unit is simplified and made more direct acting than prior circuit control means of this type. A single manually operated push button near an aisle is operated to move the units to open the aisle for human entry.